1. Field of the Invention
The present invention relates to an engine oil composition. More particularly it relates to an engine oil composition capable of reducing the friction loss in an engine and having the low friction properties not only in the early stages of use but also after it is used for a certain period of time.
2. Description of Related Arts
A lubricating engine oil for automobiles and various industrial internal combustion engines must have many properties. For example, the lubricating engine oil should (1) have good detergency, (2) be excellent in wear resistance and (3) highly stable against heat and oxidation, (4) have low oil consumption, (5) permit the engine to have a low friction loss and the like. Particularly, the property of permitting the engine to have a low friction loss as set forth in (5) above is particularly important from the viewpoint of saving energy and protecting the earth's environment from CO.sub.2 and the like.
In an attempt to meet this requirement, there have been conventionally used the technics wherein molybdenum dithiophosphate, molybdenum dithiocarbamate or like is added to the engine oil as a friction modifier (Japanese Patent Application Laid Open No.215697/1987 and Japanese Patent Publication No. 23595/1991).
However, various problems are inherent in these related arts. For example,
1) Certainly, the addition of friction modifier is effective to reduce the friction loss of the engine, but the added friction modifier is liable to be adversely affected by other additives contained in the engine oil, and there is a possibility that the effect of adding the chemical is variation-ridden and unstable.
2) Another problem is that the friction modifier is effective in the early stages of use but the effect thereof fades away comparatively soon thereafter.
Thus, the present inventor has made intensive studies with a view to finding a solution in these previous problems and developing an engine oil capable of taking an effect to steadily reduce the friction loss of an engine for a long period of time.
As the result, it has been found that said object can be achieved by incorporating into a base oil an organomolybdenum compound, a boronic succinimide and an alkaline earth metal salt of salicylic acid. The present invention has been completed on the basis of this finding.